Wireless data communication has rapidly evolved over the past decades since its conception in 1970 by Norman Abramson, who developed the world's first computer communication network, ALOHAnet, using low-cost ham-like radios. With a bi-directional star topology, the ALOHAnet system connected seven computers, deployed over four islands, to communicate with the central computer on the Oahu Island without using phone lines. In 1979, F. R. Gfeller and U. Bapst published a paper in the IEEE Proceedings reporting an experimental wireless local area network using diffused infrared communications. Shortly thereafter, in 1980, P. Ferrert reported on an experimental application of a single code spread spectrum radio for wireless terminal communications in the IEEE National Telecommunications Conference. In 1984, a comparison between infrared and CDMA spread spectrum communications for wireless office information networks was published by Kaveh Pahlavan in IEEE Computer Networking Symposium which appeared later in the IEEE Communication Society Magazine. In May 1985, the efforts of Marcus led the FCC to announce experimental ISM bands for commercial application of spread spectrum technology. Later on, M. Kavehrad reported on an experimental wireless PBX system using code division multiple access. These efforts prompted significant industrial activities in the development of a new generation of wireless local area networks and it updated several old discussions in the portable and mobile radio industry.
The first generation of wireless data modems was developed in the early 1980s by amateur radio operators, who commonly referred to this as packet radio. They added a voice band data communication modem, with data rates below 9600-bit/s, to an existing short distance radio system, typically in the two meter amateur band. The second generation of wireless modems was developed immediately after the FCC announcement in the experimental bands for non-military use of the spread spectrum technology. These modems provided data rates on the order of hundreds of kilobit/s. The third generation of wireless modem then aimed at compatibility with existing LANs with data rates on the order of several Mbit/s. Several companies developed the third generation products with data rates above 1 Mbit/s, and a couple of products had already been announced by the time of the first IEEE Workshop on Wireless LANs.
The first of the IEEE Workshops on Wireless LAN was held in 1991. At that time, early wireless LAN products had just appeared in the market and the IEEE 802.11 committee had just started its activities to develop a standard for wireless LANs. The focus of that first workshop was the evaluation of the various alternative technologies. The IEEE 802.11 standard and variants and alternatives, such as the wireless LAN interoperability forum and the European HiperLAN specification made rapid progress, and the unlicensed PCS Unlicensed Personal Communications Services and the proposed SUPERNet bands also presented new opportunities.
IEEE 802.11 is a set of standards for carrying out wireless local area network (WLAN) computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They were created and maintained by the IEEE LAN/MAN Standards Committee (IEEE 802). The 802.11 family includes over-the-air modulation techniques that use the same basic protocol. The most popular are those defined by the 802.11b and 802.11g protocols, which are amendments to the 802.11-1997 for the first wireless networking standard, but 802.11b was the first widely accepted one, followed by 802.11g and later 802.11n. Security was originally purposefully weak due to export requirements of some governments, and was later enhanced.
As a means of extending range and improving data throughput of wireless communication systems, such as those defined under the 802 standards, beam-forming techniques and MIMO circuits have been integrated with or applied to the output of wireless transmitters. Beam-forming takes advantage of directionality of an antenna array. When transmitting, a beam-former controls the phase and relative amplitude of the signal at each antenna, in order to create a pattern of constructive and destructive interference in the wavefront. When receiving, information from different sensors/antennas is combined in such a way that the expected pattern of radiation is preferentially observed. MIMO refers to “multiple-input and multiple-output”—a technology which uses multiple antennas at both the transmitter and receiver to improve communication performance. MIMO is one of several forms of smart/adaptive antenna technologies, and may be sub-divided into three main categories, pre-coding, spatial multiplexing or SM, and diversity coding:
Pre-coding is multi-layer beam-forming in the narrowest definition. In more general terms, it is considered to be all spatial processing that occurs at the transmitter. In (single-layer) beam-forming, the same signal is emitted from each of the transmit antennas with appropriate phase (and sometimes gain) weighting such that the signal power is maximized at the receiver input. The benefits of beam-forming are to increase the signal gain using constructive interference and to reduce the multipath fading effect. In the absence of scattering, beam-forming results in a well-defined directional pattern, but in typical cellular conventional beams are not a good analogy. When the receiver has multiple antennas, the transmit beam-forming cannot simultaneously maximize the signal level at all of the receive antennas, and precoding is used.
Spatial multiplexing requires MIMO antenna configuration. In spatial multiplexing, a high rate signal is split into multiple lower rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures, the receiver can separate these streams, creating parallel channels. Spatial multiplexing is a very powerful technique for increasing channel capacity at higher signal-to-noise ratios (SNR). The maximum number of spatial streams is limited by the lesser in the number of antennas at the transmitter or at the receiver. Spatial multiplexing can be used with or without transmit channel knowledge.
Diversity Coding techniques are used when there is no channel knowledge at the transmitter. In diversity methods a single stream (unlike multiple streams in spatial multiplexing) is transmitted, but the signal is coded using techniques called space-time coding. The signal is emitted from each of the transmit antennas with full or near orthogonal coding. Diversity coding exploits the independent fading in the multiple antenna links to enhance signal diversity. Spatial multiplexing can also be combined with pre-coding when the channel is known at the transmitter or combined with diversity coding when decoding reliability is in trade-off.
A transmitter, a receiver or a transceiver communicating using either single or multiple streams through multiple antenna elements (e.g. MIMO) may require multiple transmission/reception (TX/RX) chains. Both TX and RX chain are be composed of a series of circuits and/or circuit elements and may be characterized by different channel characteristics. Adjustment and/or tuning (e.g. calibration) of one or more circuits/elements of the one or more TX chains may be required in order to calibrate (e.g. equalize channel characteristics) the transceiver such that proper and efficient operation (e.g. accurate beam forming) may be achieved. The same may be true with regard to Receive (RX) Chains. Additionally, TX/RX chain element characteristics may change over time and/or due to environmental conditions during operation. Adjustment or calibration of specific elements in a TX or RX chain may not be possible during communication system operation. Compensation or equalization for TX/RX chain channel characteristics, and deviations thereof, may be achieved using a signal processing block adapted to perform compensation (e.g. pre-distortion) of signals entering and/or leaving the TX or RX chains. In order to perform such compensation, however, channel characteristics of the TX/RX chains need to be measured and/or estimated.
There remain needs in the field of wireless communication for improved methods, circuits, devices, systems and associated computer executable code for calibrating wireless communication systems.